It is maintained that a principle of indeterminacy exists in the social sciences which bears some resemblance to the Heisenberg principle in the realm of physics. In the social sciences, however, the principle is grounded not on physical interference, but on the capacity of human beings to alter their behavior on the basis of changing conceptions of their social condition, and so the contention of writers like Nagel ?that no distinct principle of explanation is involved? must be rejected. The paper concludes with a brief consideration of the causes of social indeterminacy, which suggests that the phenomenon is ineradicable